Introduction

The interactions between surfactants and water-soluble polymers in aqueous solutions have been extensively studied in the past decades because of their wide applications in various industrial processes and products, such as in pharmaceutical formulations, detergents, oil recovery fluids, cosmetic additives, and food products [1–5]. Mixing surfactants and polymers can generate many interesting functional polymorphic micro- or nano-structures, such as micelles, complexes, vesicles, precipitates, liquid crystals, gels and so on, which have a direct impact on the phase behaviors, rheological, and interfacial properties of mixed systems [6–9]. By far, many fundamental studies have been unfolded for understanding the basic mechanism of the interactions between polymer and surfactant, especially the interaction mode and aggregate behavior that occur at the molecular level in polymer-surfactant mixed systems [10–14]. In exploring polymer-surfactant interactions, two characteristic concentrations have to be mentioned to describe the aggregation process, the critical aggregation concentration (CAC), and the polymer saturation concentration (C2). The CAC corresponds to the critical surfactant concentration for surfactant-polymer complex formation which is normally smaller than the critical concentration (CMC) of the surfactant in the absence of the polymer, while C2 indicates the saturation of the polymer chains by micelle-like surfactant aggregates [9, 15]. It has been found that polymer can form mixed aggregates with surfactant, and the aggregate structure depends on the properties of polymer itself, the size of polar head and hydrophobic tail of the surfactant, the concentrations of components, ionic strength, temperature, etc. [16–19]. However, to our knowledge, the concrete interaction process between polymer and surfactants and the formation mechanism of species in mixed polymer/surfactant systems are controversial, which badly restrict their application in pharmaceutical, cosmetic, and other industrial fields. A detailed understanding of the dependence of the interaction between polymer and surfactant on the properties of surfactant and polymer, the influencing factors, such as the concentrations of components, the added salts, so that to tune the solution properties for specific applications is still a challenge.

In the past decades, numerous kinds of surfactant and polymer have been involved into the mixed polymer/surfactant systems for exploring the interaction between polymer and surfactant. Most of these studies in the liter-ature are focused on the binding interaction between ionic surfactant and oppositely charged polymer due to the strong electrostatic interaction between surfactant and polymer [20–23]. By contrast, little research has been done on the corresponding studies between ionic surfac-tants and nonionic water-soluble polymers [16, 18, 24–26]. Chauhan and co-workers [18] reported the inter-actions of the anionic surfactant sodium dodecyl sulfate (SDS) with aqueous polyethylene glycol (PEG), polyvinyl pyrrolidone (PVP), and various PEG + PVP mixtures at different temperatures by applying conductivity, density, and speed of sound techniques. The results showed that the polymers can increase micellar stability drastically and hence decrease CMC values of SDS to a large extent by interacting with SDS, and for all the studied systems, the micellization of SDS is a spontaneous, endothermic, and entropy controlled process. Banipal et al. [16] studied the effect of head groups, temperature, and polymer concen-tration on surfactant-polyethylene oxide (PEO) interac-tions using conductivity, surface tension, and viscosity methods. They found that CAC values decrease with polymer concentration and increase with temperature. However, the C2 values increase with both polymer con-centration and temperature for all surfactants, and the presence of the aromatic ring in the head group of surfactant decreases its interaction with PEO, whereas the in-creased hydrophobicity in surfactant tail strengthens its interaction with PEO. All these studies have shown that the binding interaction between ionic surfactants and non-ionic polymers displays much simpler behavior than that between ionic surfactants and oppositely charged poly-mers due to the absence of strong electrostatic forces and the hydrophobic effects play a significant role as an attractive force producing surfactant micelles that bind to the polymers below their CMC. Meanwhile, the influences of salts on neutral polymer/ionic surfactant systems have attracted increasing attention. The existing studies have shown that the added salt affects the physicochemical properties of the polymer/surfactant mixture in many ways [27–33]. Inorganic salts, such as NaCl, normally decrease the surfactant CAC, and the CAC reduction is enhanced with the increase of salt concentration [29, 30]. This is due to a preferential aggregation of surfactant molecules as a result of screening of the repulsive electrostatic repulsions between surfactant head groups by the excess cations of the added salt. The added inorganic salts also promote the binding affinity and binding ratio of the anionic surfactant to the neutral polymer [31, 32]. This has been explained by a “pseudo-polycation” model, in which a neutral polymer is coordinated with the cations of the added inorganic salt and thus favorably interacts with the anionic surfactant [31, 33]. As for the organic salts, it seems that their effects on the interaction between neutral polymers and ionic surfactants are more complicated. Lin and Hou [13] studied the roles of tetra-n-butylammonium bromide (Bu4NBr) and tetra-n-propylammonium bromide (Pr4NBr) in PVP-SDS complexation and proposed the two surfactant aggregation processes. SDS molecules associate with tetraalkylammonium bromides (TAABs) to form mixed micelles, the TAA+-SDS mixed micelles bind on the PVP chain forming the PVP-(TAA+-SDS) complex. Their work also reveals that the dominant aggregation in PVP/TAAB/SDS systems is determined by the (SDS)/(TAAB) ratio, and complexation of PVP with SDS does not occur until the(SDS)/(TAAB) ratio is larger than a specific ratio. Obviously, the interaction between polymer and surfactant can be tailed by introducing appropriate organic salts. However, the tuning mechanism is seldom explored.

The thermo-sensitive polymer has caused widespread concern for its sensitivity of the solubility to temperature. The combination of thermo-sensitive polymer and ionic surfactant is bound to have versatile properties and widely application. In this paper, we synthesized the random copolymer poly(2-(2-methoxyethoxy)ethyl methacrylate90-co-oligo(ethylene glycol) methacrylate10) (P(MEO2MA90-co-OEGMA10)) by atom transfer radical polymerization (ATRP) and characterized the thermally induced aggregation behavior of P(MEO2MA90-co-OEGMA10) in aqueous solution. The interactions between P(MEO2MA90-co-OEGMA10) and an anionic surfactant (SDS) or a cationic surfactant (DTAB) were investigated experimentally. We also examined the effects of the organic salts including Bu4NBr and Pr4NBr on the nature of the observed interactions by micropolarity measurements and 2D NOESY NMR experiments. The possible aggregation mechanism of species in P(MEO2MA90-co-OEGMA10)/organic salt/SDS mixed systems were proposed. We hope that our present study may advance our understanding on the general dynamical features and binding mechanism of polymer/ionic surfactant systems.

Materials and methods

Materials

2-(2-methoxyethoxy)ethyl methacrylate (MEO2MA, purchased from Aldrich with purity 95%) and oligo (ethylene glycol) methacrylate (OEGMA, M n  = 500 g·mol−1, from Aldrich) were purified by passing through a basic aluminum oxide column (200-300 mesh) before use. 1,1,4,7,7-pentamethyldiethylenetriamine (PMDETA, purchased from TCI with purity 98%,), methyl 2-bromopropionate (MBP, purchased from Aldrich with purity 98%), and ethanol (AR, from General-Reagent) were used as received. Copper(I) bromide (CuIBr, CP, from Sinopharm Chemical Reagent Co.,Ltd) was washed in acetic acid and ethanol and then dried in a vacuum oven. Sodium dodecyl sulfate (SDS, ACS, from Aladdin), dodecyltrimethylammonium bromide (DTAB, purchased from Aladdin with purity 99%), pyrene (purchased from Aladdin with purity 97%), Ammonium bromide (NH4Br, purchased from Alfa Aesar with purity above 99%), tetra-n-propylammonium bromide (Pr4NBr, purchased from Alfa Aesar with purity above 98%), and tetra-n-butylammonium bromide (Bu4NBr, purchased from Alfa Aesar with purity above 98%) were used as received. D2O (D-99.9%) was purchased from Aladdin. Ultrapure water (with a resistivity of 18.2 MΩ·cm) was used to prepare all aqueous solutions of surfactants, salts, and polymers.

Synthesis of P(MEO2MA90-co-OEGMA10) random copolymer

The P(MEO2MA90-co-OEGMA10) was synthesized by ATRP according to previous literature reports [34, 35]. MEO2MA (22.5 mM, 4.15 mL), OEGMA (2.5 mM, 1.16 mL), PMDETA (0.5 mM, 103 μL), and the initiator MBP (0.25 mM, 28 μL) dissolved in ethanol (10 mL) were added in a 25 mL Schlenk flask. After three freeze-pump-thaw cycles, the catalyst CuIBr (35.9 mg) was added into the degassed solution. The mixture was heated at 50 °C in an oil bath for 5 h. This reaction was stopped by opening the flask and exposing the catalyst to air. The final mixture was then diluted with ethanol and passed through a neutral alumina column (200-300 mesh) to remove the catalyst CuIBr. Then, the filtered solution was diluted with deionized water and subsequently purified by dialysis in water. Finally, after freeze-drying overnight to remove water, the random copolymer was collected (M n  = 2.20 × 104 g·mol-1, PDI = 1.63).

Micropolarity measurements

A fluorescence spectrophotometer (Hitachi, F4500) was used to measure the pyrene monomer emission spectrum of pyrene-saturated pure surfactant solution, polymer/surfactant, and polymer/salt/surfactant mixed solutions. The slit widths for excitation and emission were 5 and 2.5 nm, respectively. The excitation wavelength was 335 nm, the scan speed was 240 nm·min-1 and the scan range was from 350 to 450 nm. The I1/I3 was calculated from the intensity of peak I (at 374 nm) and peak III (at 385 nm) in the pyrene fluorescence spectrum. Pyrene-saturated water was prepared by dissolving 10.0 mg of pyrene in 1000 mL of ultrapure water, and the solution was stirred in the dark at ambient temperature for 24 h. Insoluble pyrene was filtered off, and the pyrene concentration was approximate 7.0 × 10−7 M. The pure surfactant solution, polymer/surfactant, and polymer/salt/surfactant mixed solutions were prepared using pyrene-saturated water and kept overnight at room temperature before testing. The SDS concentrations were varied from 0.025 to 100 mM, and the DTAB concentrations were varied from 0.5 to 22 mM.

Dynamic light scattering (DLS)

Particle sizes were measured using a Zetasizer Nano ZS Instrument (Malvern Instruments, UK) equipped with a 4 mW He-Ne laser (λ0 = 633 nm) and noninvasive back scattering (NIBS) detection at a scattering angle of 173°.The auto correlation function was converted into a volume-weighted particle size distribution using Dispersion Technology Software 5.06 from Malvern Instruments. The samples were stirred to mix thoroughly and kept overnight at room temperature before testing. Each measurement was repeated at least three times. The average result was used as the final hydrodynamic radius (R h ) distribution, and the estimated standard deviation of the R h values was denoted by error bars.

Turbidity measurements

The aggregation behavior of the polymer in the aqueous solutions was measured in a UV-Vis spectrophotometer (UV-2450, Shimadzu, Japan) between 200 nm and 700 nm using a cuvette with a 2 mm path length. The transmittance of the polymer solutions was monitored in a quartz cuvette (1 cm width) as a function of temperature at a wavelength of 500 nm. Heating and cooling scans were performed between 20 and 70 °C at a scanning rate of 0.1 °C·min-1.

NMR experiments

A Bruker 400 MHz NMR spectrometer instrument was used for all NMR experiments at 298.0 K. All NMR data were processed using the software package MestReNov. 6.1.0. The proton and 2D NOESY NMR spectra of the polymer, salts, and surfactant were measured using D2O or CDCl3 as the solvent, and all samples were stirred to mix thoroughly and kept overnight at room temperature before the measurement.

Results and discussion

Thermo-responsive behaviors of P(MEO2MA90-co-OEGMA10) random copolymer in aqueous solution

Using MBP as initiator and CuIBr/PMDETA as catalyst, P(MEO2MA90-co-OEGMA10) random copolymer is synthesized by ATRP of MEO2MA and OEGMA in ethanol at 50 °C. 1H NMR result (Fig. 1a) confirms the structure of P(MEO2MA90-co-OEGMA10). PMEO2MA and POEGMA are both thermo-responsive homopolymers that dissolve molecularly at 24 and 90 °C in aqueous solution, respectively, but precipitate above their LCSTs. As expected, the P(MEO2MA90-co-OEGMA10) has the similar solubility and thermo-responsive behavior in aqueous solution, and its LCST is between that of PMEO2MA and POEGMA. Figure 1b shows the temperature-dependent transmittance and hydrodynamic radii of P(MEO2MA90-co-OEGMA10). The random copolymer is molecularly soluble in water at temperatures below 38 °C, as indicated by the nearly constant transmittance at about 100%. Meanwhile, the R h values of P(MEO2MA90-co-OEGMA10) are around 7 nm under 38 °C suggesting polymer chains are molecularly dissolved, which is consistent with the transmittance results. When temperature is above 38 °C, the transmittance decreases to 0% dramatically, and the hydrodynamic radii are constantly maintained at about 1000 nm. The turbidity measurement results are coincidence with the dynamic light scattering analysis indicating the formation of the large aggregations. Obviously, the LCST of the prepared P(MEO2MA90-co-OEGMA10) is 38 °C. Furthermore, the negligible hysteresis obtained from the heating and cooling cycles of the temperature-dependent transmittance suggests that the studied thermo-responsive polymer has a good reversibility. The appreciably reversible thermal responsivity of P(MEO2MA90-co-OEGMA10) should be attributed to a delicate balance between hydrogen bonds between the copolymers and water molecules and hydrophobic interactions of polymer segments. If the balance is broken, the polymers attain a new thermodynamic equilibrium by spontaneously changing the extent of aggregation [34].

Fig. 1
figure 1

a 1H NMR spectra of P(MEO2MA90-co-OEGMA10) in CDCl3. b DLS and turbidity measurements in aqueous solutions of P(MEO2MA90-co-OEGMA10) random copolymer. The dots represent the hydrodynamic radii. The solid and open triangles represent the heating and cooling cycles of temperature-dependent transmittance

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Interaction between P(MEO2MA90-co-OEGMA10) and surfactants in aqueous solutions

SDS and DTAB are widely used in many kinds of industries because of their good emulsification property, foamability, lubrication, and dispersibility. Therefore, the SDS and DTAB are selected as objects to explore the interaction between P(MEO2MA90-co-OEGMA10) and surfactants in the present study.

P(MEO2MA90-co-OEGMA10)-SDS interactions

Micropolarity measurement using pyrene as a probe is a simple and easy way to understand the aggregation process of polymer and surfactant when exploring the interactions between polymer and surfactant. The intensity ratio of the emission of pyrene at 374 and 385 nm (I1/I3) is quite sensitive to the polarity of pyrene surroundings. The plot of I1/I3 against surfactant concentrations can be used to monitor the formation of surfactant micelles and polymer-bound surfactant aggregates in aqueous solutions [14, 36, 37]. Figure 2a presents the plots of I1/I3 ratio against surfactant concentration (CSDS) for SDS and P(MEO2MA90-co-OEGMA10)/SDS aqueous solutions. The concentration of P(MEO2MA90-co-OEGMA10) keeps constant (1 g·L−1). For pure SDS aqueous solution, the I1/I3 has no obvious changes when SDS concentration is relatively low (lower than 7 mM). However, when SDS concentration reaches about 7 mM, the I1/I3 decreases dramatically. This means SDS micelles have formed, and this concentration represents the CMC of SDS.

Fig. 2
figure 2

Plots of pyrene I1/I3 ratio against surfactant concentrations for pure surfactant and P(MEO2MA90-co-OEGMA10)/surfactant solutions. a SDS. b DTAB. The I1/I3-C surfactant curve (squared black line) of the P(MEO2MA90-co-OEGMA10)/surfactant system can be divided into three stages (I, II, and III) by two characteristic surfactant concentrations, CAC and C2

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Compared with the pure SDS system, the situation of P(MEO2MA90-co-OEGMA10)/SDS system is more complicated. It can be seen that the variation of I1/I3 with CSDS appears three different stages (I, II, and III) according to the dividing point of two characteristic concentrations, CAC and C2. These three stages represent three different modes of the interaction between P(MEO2MA90-co-OEGMA10) and SDS. [37] At lower surfactant concentration (CSDS < 1 mM), the I1/I3 keeps constant first and then decreases dramatically when SDS concentration reaches the CAC (ca. 1 mM), at which the SDS molecules begin to bind on the P(MEO2MA90-co-OEGMA10) chain to form micelle-like SDS aggregates. Further increase SDS concentration (1 mM < CSDS < 9 mM), the second stage occurs; more and more polymer-bound SDS aggregates are formed until the polymer is saturated when SDS concentration is about 9 mM (C2). The I1/I3 value in this stage varies slightly from 1.28 to 1.17, indicating that the changes of inside polarity for the studied system are almost negligible. Therefore, polarity index (I1/I3) of 1.28-1.17 can be considered as the characteristic I1/I3 value for the P(MEO2MA90-co-OEGMA10)-bound SDS complexes in the absence of added salts. In stage III (CSDS > 9 mM), the I1/I3 decreases slowly from 1.17 to ca. 1.10, showing that free SDS micelles are formed and coexist with the P(MEO2MA90-co-OEGMA10)-bound SDS complexes [14]. Based on the above analysis, the aggregation process of P(MEO2MA90-co-OEGMA10) and SDS depended on the SDS concentrations is illustrated in Fig. 3. Similar results are also observed in mixed systems of SDS/PEO and SDS/PVP [38, 39]. The hydrophobic interaction is the main driving force of the binding between nonionic polymer and ionic surfactant.

Fig. 3
figure 3

Schematic illustration of the aggregation process in P(MEO2MA90-co-OEGMA10)/SDS system

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The above analysis has proved that the surfactant SDS can interact with P(MEO2MA-co-OEGMA) by different modes depending on surfactant concentration. Our previous study has also shown that the P(MEO2MA-co-OEGMA) has the thermo-induced aggregation behavior and its LCST correlates linearly with the mole fraction of OEGMA units in the copolymer [34]. The introduction of SDS to polymer system must affect the thermo-sensitive behavior of polymer itself and further affect the application of polymer. Based on above, the influence rule of SDS on thermo-sensitive behavior of polymer and the corresponding mechanism are revealed in the present study.

Figure 4a provides the variation of R h of species existed in the mixed solutions of P(MEO2MA90-co-OEGMA10)/SDS ( concentration of polymer is 1g·L-1 and the SDS concentration is variable) as a function of temperature. The temperature at which the R h of species changes dramatically is specified as the LCST of polymer and the large aggregates are formed when the temperature above the LCST. The detailed results are listed in Table 1. It can be seen that the LCST of P(MEO2MA90-co-OEGMA10) is 38 °C when SDS is absent. However, with the addition of surfactant SDS, the LCST of the P(MEO2MA90-co-OEGMA10)/SDS mixed system increase gradually. At the same time, the R h of species in system becomes smaller and smaller after the phase transition, and the difference between the particle sizes before and after the LCST is getting less stark. Obviously, when the SDS concentration is higher than 1 mM, the particle sizes are almost equal within the studied temperature range, namely the phase transition disappear and the polymer loses its thermo-sensitive properties in the mixed system with higher SDS concentration. Our previous study has shown that the thermal responsivity of P(MEO2MA90-co-OEGMA10) is derived from the existence of both hydrogen bonds between the copolymers and water molecules and hydrophobic interactions of polymer segments [34]. For the P(MEO2MA90-co-OEGMA10)/SDS mixed system, however, the addition of SDS enhances the complexity of interactions among compositions in systems. First of all, the electrostatic repulsion among the SDS head groups cannot be neglected. The existence of electrostatic repulsion among the surfactants attached on the polymer chains hindered the aggregation of hydrophobic interactions of polymer segments. Secondly, the hydrophilic head groups of the surfactants attached on polymers have the potential of “locking water” and thus inhibit the loss of water connecting with polymers by hydrogen bonding. Obviously, the process of phase transition, namely the break of hydrogen bonding between polymers and water molecules to the aggregation of polymer due to the hydrophobic interactions of polymer segments, is retarded, and thus LCST increases. Electrostatic repulsion effect and the “locking water” effect caused by surfactant head groups affect the thermal responsivity of polymer simultaneously. When the SDS concentration is over CAC (ca. 1 mM), the electrostatic repulsion among the pre-micelles attached on the polymer chain and their “locking water” effect are strong enough to hinder the aggregation of polymer hydrophobic chains effectively. Correspondingly, the temperature-responsive aggregation phenomenon becomes less obvious. Further increase SDS concentration, the aggregation of polymer hydrophobic segments is inhibited completely, and the thermally induced phase transition behavior disappears.

Fig. 4
figure 4

Plots of R h as a function of temperature in a P(MEO2MA90-co-OEGMA10)/SDS, b P(MEO2MA90-co-OEGMA10)/DTAB, and c P(MEO2MA90-co–OEGMA10)/Bu4NBr/SDS systems. The concentration of P(MEO2MA90-co-OEGMA10) is 1 g·L-1. The SDS concentrations are varied from a 0.07 to 3.5 mM and c 0.14 to 1.05 mM, respectively, and the DTAB concentrations are varied from 0.8 to 14 mM

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Table 1 Phase transition of the P(MEO2MA90-co-OEGMA10) with different concentrations of SDS with and without Bu4NBr in the aqueous solutions
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P(MEO2MA90-co-OEGMA10)-DTAB interactions

The above analysis has shown that the properties of charged hydrophilic head group of surfactant determine the thermal responsivity of polymer to a large extent. Here, the cationic surfactant DTAB with the same alkyl chain length as anionic surfactant SDS was selected to study the interaction between polymer and surfactant. Figure 2b provides the variation of I1/I3 with DTAB concentration for DTAB and P(MEO2MA90-co-OEGMA10)/DTAB aqueous solutions. As seen in Fig. 2b, just as the situation of system containing SDS, the two characteristic concentrations, CAC (6 mM) and C2 (16 mM) are determined using micropolarity experiments. These values are much higher than those of corresponding SDS system, suggesting that DTAB has poorer capacity of self-aggregation and weaker interaction with P(MEO2MA90-co-OEGMA10) than SDS.

Similarly, the effect of DTAB concentration on the LCST of P(MEO2MA90-co-OEGMA10)/DTAB system is studied using DLS (Fig. 4b), and the detailed results are listed in Table 2. Compared with P(MEO2MA90-co-OEGMA10)/SDS system, the phase transition temperature of P(MEO2MA90-co-OEGMA10)/DTAB system increases slightly with addition of DTAB, and the hydrodynamic radius of aggregates formed above LCST is larger. It seems that the ability of DTAB to prevent the aggregation of polymer chains and “locking water” effect is weaker than that of SDS, which should be attributed to the poorer capacity of self-aggregation and weaker interaction with P(MEO2MA90-co-OEGMA10) of DTAB.

Table 2 Phase transition of the P(MEO2MA90-co-OEGMA10) with different concentrations of DTAB in the aqueous solutions
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Interactions between P(MEO2MA90-co-OEGMA10) and SDS in brine

In biological and pharmaceutical processes, polymer/surfactant mixtures are always used in salt environments. So, it is necessary to understand the roles of salt in the polymer/salt/surfactant system. In this work, the inorganic salt NH4Br, organic salts Pr4NBr, and Bu4NBr are specified to study the effect of salt on the interaction of polymer and surfactant.

Figure 5 shows the I1/I3-C SDS curves of P(MEO2MA90-co-OEGMA10)/salt/SDS systems with different salts. However, no matter what kind of salt is introduced into the P(MEO2MA90-co-OEGMA10)/SDS system, the I1/I3-C SDS curves of all systems converge at the concentration of SDS about 1 mM which correspond to the CAC of SDS. Due to the same polarity of these systems at this point, the structure of complexes attached on the polymer chains in the P(MEO2MA90-co-OEGMA10)/salt/SDS systems is similar to that of P(MEO2MA90-co-OEGMA10)/SDS system. This deduction is confirmed by the NMR experiments in the following sections.

Fig. 5
figure 5

Plots of pyrene I1/I3 ratio against C SDS for P(MEO2MA90-co-OEGMA10)/salt/SDS solutions. The salts include NH4Br, Pr4NBr, and Bu4NBr. The concentrations of salt are a 4.4 mM and b 13.2 mM. The concentration of P(MEO2MA90-co-OEGMA10) is 1 g·L−1

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It can be seen from Fig. 5, the addition of inorganic salt (NH4Br) has no obvious effect on the curves of I1/I3-C SDS, and however, the effect of organic salts is significant. Obviously, the addition of organic salts changes the typical interaction between P(MEO2MA90-co-OEGMA10) and SDS. In order to explore the influencing mechanism of organic salts on the interaction between P(MEO2MA90-co-OEGMA10) and SDS, the P(MEO2MA90-co-OEGMA10)/Bu4NBr/SDS system (CBu4NBr = 4.4 mM) was singled out for further study.

The thermo-induced phase transition behavior of P(MEO2MA90-co-OEGMA10) in P(MEO2MA90-co-OEGMA10)/Bu4NBr/SDS mixed system (CBu4NBr = 4.4 mM) was studied using DLS. Figure 4c shows the variation of R h of species in P(MEO2MA90-co-OEGMA10)/Bu4NBr/SDS solutions with temperature. Similarly, the concentration of P(MEO2MA90-co-OEGMA10) keeps constant (1 g·L−1). The detailed results are listed in Table 1.

Figure 4c reveals that the P(MEO2MA90-co-OEGMA10) has the same thermally induced aggregation behavior in P(MEO2MA90-co-OEGMA10)/Bu4NBr/SDS solution as in P(MEO2MA90-co-OEGMA10)/SDS aqueous solution. However, the addition of Bu4NBr changed the LCST of P(MEO2MA90-co-OEGMA10) significantly. The LCST of P(MEO2MA90-co-OEGMA10) in system with Bu4NBr is much lower than that in system without Bu4NBr. Meanwhile, the hydrodynamic radius of species formed in system with Bu4NBr is much larger than that of in system without Bu4NBr when the temperature is above the corresponding LCST. The addition of Bu4NBr can hinder the combination of P(MEO2MA90-co-OEGMA10) and SDS by the electrostatic interaction and thus result in the smaller resistance among the polymer chains in the aggregation process. As a result, compared to that in aqueous solution, the association of P(MEO2MA90-co-OEGMA10) increases; lower LCST and larger aggregates are observed.

Based on the above experiments, the relations between LCST of different systems and surfactant concentrations were established, as shown in Fig. 6. The simple linear relations between LCST and surfactant concentration for all systems were found and can be precisely described by the following equations:

$$ {\mathrm{LCST}}_1=40.29+19.59{\mathrm{C}}_{SDS}\kern0.28em \left(0\kern0.28em mM<{\mathrm{C}}_{SDS}<1.05\kern0.28em mM,\mathrm{P}\left({\mathrm{MEO}}_2{\mathrm{MA}}_{90}{\textstyle \hbox{-}}\mathrm{c}\mathrm{o}{\textstyle \hbox{-} }{\mathrm{OEGMA}}_{10}\right)/\mathrm{S}\mathrm{D}\mathrm{S}\right) $$
(1)
$$ {\mathrm{LCST}}_2=40.39+2.35{\mathrm{C}}_{\mathrm{DTAB}}\left(0\kern0.28em mM<{\mathrm{C}}_{\mathrm{DTAB}}<10\kern0.28em mM,\mathrm{P}\left({\mathrm{MEO}}_2{\mathrm{MA}}_{90}{\textstyle \hbox{-}}\mathrm{c}\mathrm{o}{\textstyle \hbox{-} }{\mathrm{OEGMA}}_{10}\right)/\mathrm{DTAB}\right) $$
(2)
$$ {\mathrm{LCST}}_3=38.32+9.38{\mathrm{C}}_{SDS}\kern0.28em \left(0\kern0.28em mM<{\mathrm{C}}_{SDS}<1.05\kern0.28em mM,\mathrm{P}\left({\mathrm{MEO}}_2{\mathrm{MA}}_{90}{\textstyle \hbox{-}}\mathrm{c}\mathrm{o}{\textstyle \hbox{-} }{\mathrm{OEGMA}}_{10}\right)/{\mathrm{Bu}}_4\mathrm{N}\mathrm{B}\mathrm{r}/\mathrm{S}\mathrm{D}\mathrm{S}\right) $$
(3)
Fig. 6
figure 6

Standard curve of the measured LCST as a function of surfactant concentration (CSDS or CDTAB) for P(MEO2MA90-co-OEGMA10) in aqueous solutions with and without salt. 0 mM < C SDS < 1.05 mM; 0 mM < CDTAB < 10 mM

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Obviously, the slopes of these lines reveal the strength of the interaction between P(MEO2MA90-co-OEGMA10) and surfactant in aqueous solutions with and without salt. According to these equations, it can be seen that the LCST of the system containing P(MEO2MA90-co-OEGMA10) could be tuned to the expected temperatures by simply introducing slightly surfactants, salts, or both of them to the system.

Figure 7 illustrates the changes of I1/I3 with SDS concentration of P(MEO2MA90-co-OEGMA10)/Bu4NBr/SDS system. Compared with the situation of system without Bu4NBr (Fig. 2a), we can see that the I1/I3-CSDS curve for P(MEO2MA90-co-OEGMA10)/Bu4NBr/SDS (CBu4NBr = 4.4 mM) system has been divided into four distinct stages (stages A, B, C, and D). With the increase of SDS concentration, the I1/I3 undergoes constant, decrease, constant, and decrease again, respectively. In the first stage, namely stage A (C SDS < 0.08 mM), I1/I3 is maintaining a constant value of about 1.55, indicating that no distinct hydrophobic microdomains are formed in the solution. Almost all species in solutions, including SDS, Bu4NBr, and P(MEO2MA90-co-OEGMA10) exist in the state of monomer, and no interaction among polymer, SDS, and Bu4NBr is explored. With the increase of surfactant concentration (0.08 mM < CSDS < 1.0 mM), as can be seen in stage B, I1/I3 deceases dramatically. It is worth mentioning that the concentration at which the I1/I3 start decrease significantly (CSDS = 0.08 mM) corresponds to the CMC of the Bu4N+-SDS mixed solution [36]. This implies that the Bu4N+-SDS mixed micelles should be formed under this condition due to the electrostatic interaction and hydrophobic interaction between SDS and Bu4NBr. Furthermore, it is observed that the I1/I3 values are located in the range of 1.55 to 1.32, which is apparently different from the characteristic polarity index value for the P(MEO2MA90-co-OEGMA10)-bound SDS complexes (the I1/I3 values are within the range of 1.28 to 1.17). It looks like that the interaction is not obvious in this stage.

Fig. 7
figure 7

Plot of the pyrene I1/I3 ratio of P(MEO2MA90-co-OEGMA10)/Bu4NBr/SDS system (CBu4NBr = 4.4 mM) as a function of the SDS concentrations

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Further increase of the SDS concentration (1.0 mM < CSDS < 10 mM), as shown in stage C, the values of I1/I3 keep almost constant. In principle, if there are two or more different hydrophobic microdomains with different polarities in the aqueous phase, pyrene molecules will be partitioned into these microenvironments [40, 41]. So the observed I1/I3 values should be considered as a weighted average of the polarity in these different hydrophobic microdomains. The slight change of I1/I3 from 1.32 to 1.30 indicates that the microdomains of new species should have the similar polarity to that of the Bu4N+-SDS mixed micelles once they are formed in solutions. We speculate maybe the Bu4N+-SDS mixed micelles are bonding to the polymers gradually during the course of increasing SDS concentration. The existence of new species of P(MEO2MA90-co-OEGMA10)-bound Bu4N+-SDS complexes has been confirmed by the NMR experiments in the following sections. It can be concluded that Bu4N+-SDS mixed micelles coexist with P(MEO2MA90-co-OEGMA10)-bound Bu4N+-SDS complexes in the solutions in stage C.

In Fig. 5a, b, the level in the I1/I3 begins and ends at the same SDS concentration (ca. 1.0 and 10 mM), regardless of the concentration of Bu4NBr. Consequently, in the presence of Bu4NBr, the adsorption of SDS molecules onto P(MEO2MA90-co-OEGMA10) is directly related to the SDS concentration. When the SDS concentration is higher than 10 mM, as can be seen in stage D (10 mM < CSDS < 100 mM), the I1/I3 decreases dramatically until to about 1.1 that coincide with the characteristic polarity index of 1.28-1.17 for the P(MEO2MA90-co-OEGMA10)-bound SDS complexes when the SDS concentration is over 50 mM. This reveals that the average polarity of the hydrophobic microdomains in region D for the studied system is similar to that of the hydrophobic microdomains produced by free SDS micelles and P(MEO2MA90-co-OEGMA10)-bound SDS complexes. Obviously, the higher of the SDS concentration (higher than 50 mM), the less obvious of the Bu4NBr effect on the P(MEO2MA90-co-OEGMA10)/SDS system. Correspondingly, the interaction behavior of the P(MEO2MA90-co-OEGMA10)/Bu4NBr/SDS system is almost equivalent to that of P(MEO2MA90-co-OEGMA10)/SDS system when SDS concentration is higher.

The above analysis has shown that the species formed in the P(MEO2MA90-co-OEGMA10)/Bu4NBr/SDS system are variable with SDS concentration. The 2D NOESY experiments are used to further identify the formation and microstructure of these species. The 2D NOESY NMR is a useful technique for elucidating the microstructure of a polymer-surfactant complex, and it has been applied to investigate polymer penetration into surfactant aggregates in aqueous media [10–12, 14, 42]. The 1H NMR spectra of SDS, P(MEO2MA90-co-OEGMA10), and Bu4NBr in D2O are shown in Fig. 8. Referring to the results obtained by micropolarity measurements (Fig. 7), three samples which characterize different stages of P(MEO2MA90-co-OEGMA10)/Bu4NBr/SDS system (SDS concentrations are 0.8, 8, 20 mM, respectively) are selected for the 2D NOESY analysis; the results have been shown in Fig. 9.

Fig. 8
figure 8

1H NMR spectra of a SDS, b P(MEO2MA90-co-OEGMA10), and c Bu4NBr in D2O

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Fig. 9
figure 9

2D NOESY spectra of P(MEO2MA90-co-OEGMA10)/Bu4NBr/SDS solutions in D2O.CSDS = a 0.8 mM (region B). b 8 mM (region C). c 20 mM (region D). The concentrations of P(MEO2MA90-co-OEGMA10) and Bu4NBr are 1 g·L−1 and 4.4 mM, respectively

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Figure 9a provides the information on the microstructure of species formed in P(MEO2MA90-co-OEGMA10)/Bu4NBr/SDS system when SDS concentration is 0.8 mM. The clear cross-peaks between Bu4N+ N1 and N2 protons and SDS S3–11 protons indicate the formation of the Bu4N+-SDS mixed micelles, and the Bu4N+ ions are not only attached to the micellar surface but also inserted into the micellar interior of mixed micelles in region B.

In Fig. 9b (CSDS = 8 mM, region C), the patterns of intramolecular and intermolecular cross-peaks arising from Bu4N+ ion and SDS are identical to those shown in Fig. 9a, indicating the Bu4N+-SDS mixed micelles still exist in region C. In addition, the weak NOE cross-peaks between the P(MEO2MA90-co-OEGMA10) protons and the SDS protons (S3–11) shown in Fig. 9b indicate that the P(MEO2MA90-co-OEGMA10) only has weak interaction with SDS. Thus, for the P(MEO2MA90-co-OEGMA10)/Bu4NBr/SDS system, the lack of direct NOE cross-peaks between P(MEO2MA90-co-OEGMA10) and SDS suggests that the P(MEO2MA90-co-OEGMA10) chain does not penetrate into the region occupied by the SDS alkyl chains and only tangle through the surface of P(MEO2MA90-co-OEGMA10)-bound SDS complexes. This is similar to that complexes formed in the P(MEO2MA90-co-OEGMA10)/SDS system. So, there is a coincident point in Fig. 5. In the presence of Bu4NBr, due to the NOESY data, it can be concluded that, free Bu4N+-SDS mixed micelles and P(MEO2MA90-co-OEGMA10)-bound Bu4N+-SDS complexes coexist in the solution of P(MEO2MA90-co-OEGMA10)/Bu4NBr/SDS when SDS concentration is located in region C. It should be noted that no cross-peaks between P(MEO2MA90-co-OEGMA10) and Bu4NBr can be observed, suggesting that the P(MEO2MA90-co-OEGMA10) chain is not in proximity to the Bu4N+ ions even though the Bu4N+ ions are absorbed on the P(MEO2MA90-co-OEGMA10)-bound Bu4N+-SDS complexes.

The 2D NOESY spectrum in Fig. 9c shows that all SDS protons have cross-peaks with P(MEO2MA90-co-OEGMA10) protons when SDS concentration is 20 mM in the P(MEO2MA90-co-OEGMA10)/Bu4NBr/SDS system. Specifically, the presence of cross-peaks between P(MEO2MA90-co-OEGMA10) and S12 suggests that the P(MEO2MA90-co-OEGMA10) chain penetrates into the core of the polymer-bound Bu4N+-SDS complexes. The positive NOE correlation among the protons of the Bu4N+ ion and clear cross-peaks between Bu4N+ N1 and N2 protons and SDS S3–11 protons in Fig. 9c indicates that there must be free Bu4N+-SDS mixed micelles exist in region D. Obviously, the P(MEO2MA90-co-OEGMA10) chain penetrates into the core of the polymer-bound Bu4N+-SDS complexes and also coexists with free Bu4N+-SDS mixed micelles in stage D. Similar results are also found in the P(MEO2MA90-co-OEGMA10)/Pr4NBr/SDS system (Figure S1 and S2). According to the analysis of the micropolarity and 2D NOESY NMR measurements, the schematic illustration of the aggregation process in P(MEO2MA90-co-OEGMA10)/Bu4NBr/SDS system is shown in Fig. 10.

Fig. 10
figure 10

Schematic illustration of the aggregation process in P(MEO2MA90-co-OEGMA10)/Bu4NBr/SDS system (CBu4NBr = 4.4 mM)

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Conclusions

The thermally induced aggregation behavior of P(MEO2MA90-co-OEGMA10) in aqueous solution is studied and the interaction between P(MEO2MA90-co-OEGMA10) and SDS or DTAB in aqueous solutions with and without salt is explored by using DLS, pyrene fluorescence spectroscopy, and 2D NOESY NMR technology. The influence rule of surfactant on thermo-sensitive behavior of polymer and the corresponding mechanism are revealed. The results have shown that the P(MEO2MA90-co-OEGMA10) has an appreciably reversible thermal responsivity (LCST = 38 °C) in aqueous solution due to a delicate balance between hydrogen bonds between the copolymers and water molecules and hydrophobic interactions of polymer segments. The addition of surfactants changes the aggregation behavior of polymers by interaction with polymers. The self-aggregation of polymer chains is hindered due to the strong electrostatic repulsion and the “locking water” effect caused by surfactant head groups and thus leads to the increase of LCST with the increase of surfactant concentration for the system of polymer/surfactant. When the surfactant concentration is higher than CAC, the aggregation of polymer hydrophobic segments is inhibited completely, and the thermally induced phase transition behavior disappears.

The organic ions Bu4N+ and Pr4N+ retard the interaction between P(MEO2MA90-co-OEGMA10) and SDS. In P(MEO2MA90-co-OEGMA10)/organic salt/SDS system, the Bu4N+-SDS or Pr4N+-SDS mixed micelles are formed first, then the mixed micelles bind on the polymer chain forming the polymer-bound Bu4N+-SDS or Pr4N+-SDS complexes. And the P(MEO2MA90-co-OEGMA10) chain in the P(MEO2MA90-co-OEGMA10)-(Bu4N+ or Pr4N+-SDS) complex is located only on the complex surface and far away from the surface-attached Bu4N+ or Pr4N+ ions. Furthermore, the quantitative relations between LCST of different systems and surfactant concentrations were established. The LCST of the system containing P(MEO2MA90-co-OEGMA10) can be adjusted by tailoring the surfactant concentration, salt concentration of the system. The present results not only provide detailed information on the interaction between polymer and ionic surfactants as well as the explicit role of organic salts in influencing the interaction, but also advance our understanding on the general dynamical features and binding mechanism of polymer/ionic surfactant systems.